1. Field of the Invention
The present invention relates in general to a multiphase electrorheological fluid, and more particularly to an electrorheological fluid comprising a suspension of solid particles and emulsion liquid droplets dispersed in a continuous oil phase, in which the emulsion liquid droplets are immiscible with the continuous oil phase. Thus, this electrorheological fluid has enhanced properties as compared to the conventional electrorheological fluid composed of a solid particle suspension in oil.
2. Description of the Prior Art
An electrorheological fluid has a fast response time of a few milliseconds and can be adjusted in its viscosity in response to a variation in electric field. Thus, it can be applied in various fields, such as electrically working active suspension systems, valves, brakes, artificial joints and so on.
Electrorheological phenomenon is associated with a variation in Theological properties of a suspension which occur when an external electric field is applied. The electrorheological fluid shows the same behavior as the usual Newtonian fluid in the absence of the electric field, but it is solidified in the presence of the electrical field and shows a strong flow resistance. A great variation in viscosity occurring in the electrorheological fluid is due to a variation in microstructure of a suspension. The application of the electrical field to a static suspension results in rearrangement of particles in the suspension by the polarization phenomenon occurring within the particles or on their surface, and forms a fibril structure connecting electrodes to each other. Where a strain is applied to the fibril structure of the particles perpendicular to the direction of electric field, the fibril structure is distorted. Energy consumed by this strain causes an increase in viscosity of the suspension. In this case, yield stress of the suspension is increased as electric field strength is increased. Meanwhile, if the applied shear stress is higher than the yield stress of the fluid, the fluid has fluidity. The electrorheological fluid responds to the electric field in a highly fast time of about 10xe2x88x923 seconds, and this response is reversible, so that the electrorheological fluid can be employed as an excellent medium to transfer electrical signals to mechanical devices. There has been proposed many mechanical devices using the electrorheological fluid, including the clutches, high speed valves, and vibration-controlling active suspension systems.
Many kinds of dispersion mediums and particles are disclosed as components of the electrorheological fluid (U.S. Pat. Nos. 3,397,147; 4,483,788; 4,502,973; and 4,668,417). It is generally known that the electrorheological fluid contains a small amount of water adsorbed on particles dispersed therein (less than 10% by weight relative to the particle weight). Thus, by virtue of the ion polarization phenomenon occurring upon the application of the electric field, the electrorheological fluid exhibits the electrorheological effect by the formation of a chain structure or by the formation of a water-crosslinked structure between the particles.
The electrorheological activity of this fluid significantly depends on a variation in water content of the fluid. If this fluid is free of water, it disadvantageously loses its electrorheological activity and can be not used at high temperature. The fluid free of water also has drawbacks in the engineering view in that it results in high abrasion of a machine and is limited in its working temperature. It was recently reported that suspensions having completely dried inorganic or polymeric particles dispersed therein also have occurred the electrorheological phenomenon. In these suspensions, the dispersed particles are a semiconductor in their electrical property, and also the polarization phenomenon on the application of the electric field occurs by the migration of charge carriers by virtue of inherent physical and chemical properties of the particles other than occurring by water. U.S. Pat. No. 5,417,874 to Carlson et al. discloses an electrorheological fluid using inorganic particles of a crystalline lattice structure, which fluid can be worked at a temperature range of 25 to 150xc2x0 C. However, the disclosed electrorheological fluid has a drawback in that the dispersed particles are high in their density and thus are easily settled.
Representative polymeric particles dispersed in the non-aqueous electrorheological fluid include polyaniline particles (See, xe2x80x9cThe Electrorheological Properties of Polyaniline Suspensionsxe2x80x9d, J. Colloidal and Interface Science, Vol. 126, No.1, April 1990, pp. 175-188). European Patent Publication A 394,005 discloses an electrorheological effect of a suspension of 30% by volume polyaniline dispersed in a silicone oil. U.S. Pat. Nos. 5,595,680 and 5,437,806 describe non-aqueous electrorheological fluids using polyanilines and derivatives thereof polymerized from aniline monomers and a mixture of aniline monomers and various monomers.
A dispersion medium of the electrorheological fluid must have an electrically insulating property and may contain a surfactant to improve its stability. An effective dispersion medium generally needs to have a good dispersibility, a low viscosity and electrical conductivity, a high boiling point, a low freezing point, a chemical stability, and a high dielectric strength. U.S. Pat. No. 4,687,589 discloses physical property values required in the dispersion medium.
Halogenated oil is great in its specific gravity and less in its particle-settling degree, as compared to the conventionally used silicone oil. Also, the halogenated oil may be increased in its electrorheological activity as compared to the silicone oil, but a precious mechanism for this increase is not known. In the case where additives such as surfactant are included in the halogenated oil, their concentration needs to be limited to such a low degree that it is present only on the particle surface. A chain structure formed by the electric field is necessarily accompanied with the exhibition of the electrorheological phenomenon, and the shape and thickness of the chain depend on the physical and chemical properties of the components of the fluid. The performance and stability of electrorheological fluids developed up to now are difficult to meet a stress transfer property required in practical devices, and these fluids thus need to be improved in their performance and stability. Yield stress, a representative property, depends on the applied electric field strength and the particle volume fraction. To achieve a greater yield stress at a realizable electric field strength, increasing the particle volume fraction is effective. However, this particle volume fraction cannot disadvantageously exceed any maximum value, which is varied depending on a viscosity of the dispersion medium, and a shape and surface property of the particles. Moreover, an excessively concentrated dispersion system is excessively high in its viscosity in the absence of the electric field, as well as in the electric current leakage that causes the dielectric breakdown on the application of large electric field. For this reason, this dispersion system is disadvantageous in that it has insufficient controllability and stability. Thus, a new electrorheological fluid is required that is not excessively high in its particle concentration while having a high yield stress and an excellent stability.
In addition to the particles suspended in the insulating dispersion medium, an emulsion liquid droplet also undergoes an electrostatic interaction in the presence of the electric field. An article by Pan et al. has reported electrorheological properties of an emulsion under the electric field (Pan et al., xe2x80x9cCharacteristics of Electrorheological Response in an Emulsion Systemxe2x80x9d, J. Colloidal and Interface Science, Vol. 195, No. 1, 1997, pp.101-113).
We have found that an electrorheological fluid comprising a suspension of particles and emulsion liquid droplets dispersed in a continuous oil phase particles exhibits a highly stable microstructure in the form of a chain structure which is compositely formed by the particles and the emulsion liquid droplets. Based on this discovery, we have perfected the present invention.
It is therefore an object of the present invention to provide a new multiphase electrorheological fluid, which contains a continuous liquid phase and a dispersed liquid phase at optimally established viscosity and electrical property conditions, and can be thus highly increased in its yield stress without excessively increasing a volume fraction of particles dispersed in the continuous oil phase.
According to the present invention, a multiphase electrorehological fluid is provided. First liquid phase component is used as a continuous oil phase. Particles as well as second liquid phase component are dispersed in the continuous oil phase simultaneously while this emulsion drops are immiscible with the continuous oil phase. The dispersed liquid droplets have higher electrical conductivity and dielectric constant than those of the continuous oil phase.